The present invention relates generally to seals for tank bottoms and more particularly to corner locks for efficient and effective installation of tank bottom seals.
In the construction and rehabilitation of flat bottom tanks with a vertical cylindrical shell, it is sometimes necessary or desirable to add a second, upper tank bottom above the first lower tank bottom. The lower tank bottom can be sealed to minimize leakage from the tank through perforations that may develop in the upper tank bottom. Further, the space between the upper and lower tank bottoms can be backfilled with soil, sand, gravel or concrete depending on the load bearing requirements of the tank bottom.
A material commonly used to line the lower bottom of the tanks is high density polyethylene (HDPE). It provides the desired sealing properties while having the durability to withstand construction worker foot-traffic and the impact of back-fill material. HDPE also provides enough flexibility in the seal to conform to irregular tank bottom surfaces during construction and back filling. It can also be installed during adverse weather conditions and no special tank surface preparation is necessary.
A number of liner installation techniques are in use which have satisfactory results but which have numerous quality assurance problems that are not immediately apparent since seal effectiveness may not be challenged until many years after installation.
One such technique uses sheets of HDPE on the tank bottom and prevents shifting of the sheets by using percussion pins to fix the sheet to the tank bottom. Obviously, such pins can damage the existing structure and any holes in the HDPE reduce its effectiveness as a seal. Epoxy can be used to cover the pin and adjacent HDPE. The effectiveness of the epoxy patching is unknown since experience indicates that some types of epoxy will not adhere well to HDPE. Thus, this technique can result in a perforated seal that must rely in part on concrete back-fill for effectiveness. Further, a flat seal such as this only covers the bottom of the tank which is unable to prevent run-off at the tank corners where the shell meets the tank bottom.
Another technique for lining tank bottoms uses flat sheets on the tank bottom in addition to a vertical component on the tank shell so that the seal can retain at least some fluid which may leak through the upper tank bottom. This second technique requires the welding of short studs around the tank shell. Vertical sheets of HDPE are secured to the studs using nails. Again, nail holes in the HDPE may or may not be effectively sealed using epoxy and further damage to the tank structure can result from the welding process. A disadvantage of using studs on the vertical walls is that the seal is spaced out from the tank shell where it is vulnerable to punctures. Yet another disadvantage in this techniques is that the corner where the vertical and horizontal pieces of HDPE meet must be sealed. Plastic welding techniques are well known and can be used, but it is difficult to effectively weld such a corner in the field.
A third technique is used which includes the use of a sealing paint on the vertical shell surfaces which can overlay the horizontal seal on the tank bottom. The primary drawback of this technique is that the corner seal is typically unsatisfactory since even minor shifting or movement in the horizontal sheet material can break the paint seal at the corner. Fiberglass may be similarly used to adhere to both the tank shell and the tank bottom, but this requires sandblasting of the tank surfaces and favorable weather conditions.